Marathon is not only from Bungie which still has a great history with Xbox and the studio which gave many Xbox fans their favourite IP, it also doesn't hurt to let people know that it also comes to Xbox. Something people might forget about because Bungie is owned by Sony.
And since it's Sony's studio which pretty much works under a similar concept as MS's studios (release their games on "all" platforms) it is pretty obvious that Phil is a fan of that







